Cellular growth, proliferation and development are regulated in proportion to biosynthesis and assembly of active ribosomes. Expression of genes encoding ribosomal proteins and ribosomal RNA (rRNA) involves most of the machinery and processes of eukaryotic gene expression. The main goal of this proposal is to study the function of Drs1p, a nucleolar protein and putative RNA helicase in yeast that is necessary for production of 25S rRNA and assembly of 6OS ribosomal subunits. It was suggested that Drs1p might be required to facilitate rearrangements of RNA structure to facilitate processing and assembly of rRNA with r-proteins into ribosomes.To investigate whether in fact Drslp facilitates rearrangements in the secondary structure of rRNA or snoRNAs during pre-rRNA processing: (I) the steps in pre-rRNA processing where Drslp is involved will be defined at higher resolution by northern blotting and primer extensions. (2) A search for RNA ligands of Drs lp in rRNA precursors will be performed by using genetic and biochemical approaches. (3) ATPase or RNA helicase activity will be assayed using the RNA ligand candidates identified by the genetic screens. Ribosome assembly involves many processes that are central to cellular function: RNAprotein and protein-protein interactions, transport between the nucleus and the cytoplasm, and formation of multimeric complexes. Studies in ribosome biogenesis will increase our understanding of many important cellular functions and global regulatory networks involved during cell growth and development.